1. FIELD OF THE INVENTION
The present invention relates to an improvement of a glass body comprising an alkali-containing glass substrate and a silica layer formed thereon to prevent diffusion of alkali metal ions from the glass substrate.
2. DESCRIPTION OF THE PRIOR ART
Glass plates as transparent material are chemically stable and have superior surface hardness, and they are durable at a high temperature at a level of 500.degree. to 700.degree. C. and superior in their electric insulating property and optical property. Accordingly, they are widely used not only as window materials for architectural structures, vehicles or aircrafts, but also for optical elements or electric or electronic elements. Recently, electroconductive glass plates in which an electroconductive coating is formed on a glass plate, have been used for display devices such as liquid crystal device, an electrochromic device and an electric field luminance device, or an amorphous solar cell substrate. As a glass substrate for these electroconductive glass plates, a soda lime silica glass plate is most commonly used as it is relatively inexpensive. However, the soda lime silica glass plate contains from about 10 to 20% by weight of an alkali metal component such as sodium or potassium, and has a drawback such that when used for an extended period of time, alkali metal ions tend to diffuse from the glass substrate to its surface, thereby leading to degradation of the property of the coated electroconductive layer. For instance, it is likely that white turbidity will be formed in the electroconductive layer of the electroconductive glass plate, the transparency of the layer will be lowered, the electric resistance of the electroconductive layer will be increased or the physicochemical durability will be reduced.
Furthermore, in the case of a liquid crystal display device, an oxidation-reduction reaction will be caused at the surface of the display electrode by the alkali metal ions diffused from the glass, whereby the transparent electrode material i.e. an indium oxide layer (ITO layer) or tin oxide layer (Nesa layer), will undergo a property change and the liquid crystal itself will undergo electrolysis and will be degraded. In the case of an electrochromic device, the electrode will be worn for the same reasons and the electrochromic material such as tungsten oxide or molybdenum oxide will be electrically corroded and undergo a property change, whereby the device will be degraded. Likewise, in the case of an electric field luminance device, the alkali metal ions leached out of the glass surface penetrate through the electroconductive layer and diffuse into the phosphor material, whereby the luminance efficiency and the colour of the luminance will be changed. Further, in the case of an amorphous solar cell, the alkali metal ions leached out of the electrodes are likely to diffuse into the amorphous silicon thereby to reduce the conversion efficiency.
Further, an alkali-containing glass material such as soda lime silica glass has a tendency such that when subjected to a high temperature treatment, the alkali metal ions readily move, and accordingly, there is a drawback such that during the high temperature treatment for the production of electroconductive glass materials or various coated glass materials, the alkali ions readily diffuse whereby the properties of the electroconductive layers or various coated layers will be degraded.
In general, there are three different methods for solving the above mentioned drawbacks of glass plates. One of them is to use a glass plate having a composition which is free from diffusion of alkali metal ions, such as silica glass, high silica glass (Vycor glass), non-alkali-containing aluminum silicate glass (such as CGW #7059), lower-alkali borosilicate (such as Pyrex glass). However, these glass materials are expensive and not necessarily readily available, and they are inferior in the surface smoothness, and accordingly its surface is required to be repolished. In fact, in an extreme case, they are not available in a thin glass plate, and therefore it will be necessary to grind them to obtain a plate having a thickness of a few millimeters and further to polish the plate to obtain a glass plate having a thickness of 1 mm. This is undesirable with a view to conservation of the material resource and energy.
The second method is to preliminarily remove or reduce the alkali component from the surface layer of soda lime silica glass. There are proposed a method in which the surface layer is brought in contact with sulfur at a high temperature, a method in which the glass material is heated in vacuum to a high temperature of at least 300.degree. C. and a direct current electric field is applied to drive the Na.sup.+ ions toward the opposite side to provide alkali-deficient surface on which the ITO (Nesa) coating is applied, or a method in which the glass material is boiled in an acid such as hydrochloric acid or sulfuric acid. This method is disadvantageous in that it takes a long time for the operation and the reproducibility is inadequate.
The third method is to form on the surface of soda lime silica glass a certain thin layer for preventing diffusion of alkali metal ions. It is usual for use a silica layer. The reason for using an silicon oxide layer (e.g. SiO.sub.2 layer) for the prevention of the diffusion of alkali metal ions is that the layer is amorphous and when another thin layer such as an electroconductive layer is to be formed thereon, it is possible to form substantially the same layer as formed on glass, and the reflactive index of the silicon oxide layer is similar to that of glass although it is slightly less than the reflactive index of glass. Further, the silicon oxide layer usually is transparent against a wider range of lights than the glass plate, and accordingly the transparency of the glass plate will not thereby be impaired. The silicon oxide layer mentioned above is meant in a broad sense, and more specifically, it includes a pure silicon oxide layer and a silicon oxide layer containing a proper amount of impurities, for instance, a silicon oxide layer incorporated with a small amount of boron or phosphorus to improve the ability to prevent the diffusion of alkali metal ions. As representative methods for forming such as an alkali diffusion-preventing silicon oxide layer, there may be mentioned a method in which a silicon oxide layer of e.g. pure SiO.sub.x (0&lt;x.ltoreq.2) is formed by sputtering, vacuum vapour deposition, CVD process or ion-plating process under a high vacuum condition so that the layer is formed as dense as possible to increase the ability to prevent the alkali diffusion, a method in which a pure silicon oxide layer as above is formed by a sol/gel process, and a method in which a boron-containing silicon oxide layer or a phosphorus-containing silicon oxide layer is formed by a sol/gel process whereby an additive such as boron or phosphorus can readily be incorporated.
Alkali diffusion-preventing silicon oxide layers formed by such various methods have certain effectiveness for preventing the diffusion of alkali metal ions although the degree of the effectiveness varies. However, the effectiveness is not usually sufficient, and there is a further drawback that the property varies considerably depending upon the manner and the conditions for their preparation.